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Function Parameters

Beginner~18 min

TypeScript provides flexible parameter options: optional, default, and rest parameters, all with type safety.

Optional Parameters

Use ? to make parameters optional. Optional parameters can be undefined:

Output 
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Important: Optional parameters must come AFTER required parameters. 
// ✗ Wrong
function greet(lastName?: string, firstName: string) { }

// ✓ Correct
function greet(firstName: string, lastName?: string) { }

Default Parameters

Default parameters have a fallback value if not provided:

Output 
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Tip: Default parameters are automatically optional. You don't need to add ?

Optional vs Default Parameters

Feature Optional (?) Default (=)
Value if omitted undefined Default value
Type includes undefined Yes No
Must check for undefined Yes No
Position requirement After required Anywhere

Rest Parameters

Rest parameters collect multiple arguments into an array:

Output 
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Rest Parameter Rules:
  • Must be the LAST parameter
  • Only ONE rest parameter allowed
  • Type must be an array

Common Mistakes

1. Wrong Parameter Order

// ✗ Wrong - optional before required
function create(id?: number, userName: string) { }

// ✓ Correct
function create(userName: string, id?: number) { }

2. Multiple Rest Parameters

// ✗ Wrong - only one rest parameter allowed
function combine(...nums: number[], ...strs: string[]) { }

// ✓ Correct - use union type
function combine(...items: (number | string)[]) { }

Summary

  • Optional: param? - can be undefined
  • Default: param = value - has fallback
  • Rest: ...params - collects multiple args
  • Optional/rest must come after required parameters

What's Next?

Next: Function Overloading for multiple function signatures!

Previous ← Function Types
Next Overloading →